A communications device such as a cable modem termination system (CMTS) is typically provided at a headend or hub site of a broadband network for providing high speed data services such as Internet, Voice over Internet Protocol, or digital video services to subscribers of a cable TV operator or to like customers. The CMTS hosts downstream and upstream ports and contains numerous receivers, each receiver handling communications between hundreds of end user network elements connected to the broadband network. Examples of network elements include cable modems, set top boxes, televisions equipped with set top boxes, Data Over Cable Service Interface Specification (DOCSIS) terminal devices, media terminal adapters (MTA), embedded media terminal adapter (eMTA), and the like. An example of a CMTS is the Motorola Broadband Service Router 64000 (BSR 64000).
Data Over Cable Service Interface Specification (DOCSIS) is a cable modem standard used for transferring data over a cable TV network. The CMTS carries IP traffic (downstream traffic) destined for cable modems and like network elements. The downstream traffic is carried in IP packets encapsulated in MPEG transport stream packets carried on data streams that are typically modulated onto a TV channel.
DOCSIS specifies that cable modems and like network elements obtain upstream bandwidth according to a request/grant scheme. A cable modem sends a bandwidth allocation request when subscriber network devices need to send traffic upstream into the network. The CMTS grants these requests using bandwidth grant messages. Thus, the CMTS must arbitrate bandwidth among a plurality of network elements such as set top boxes and cable modems configured for bi-directional communications. Upstream data (data from cable modems to the headend or Internet) is carried in Ethernet frames encapsulated inside DOCSIS frames using time-division multiple access (TDMA) sharing mechanisms.
The CMTS can be used to serve customers on a Hybrid Fiber-Coaxial (HFC) broadband network, a Radio Frequency over Glass (RFoG) broadband network, or a mixed HFC and RFoG network. Traditionally, cable TV operators have used HFC broadband networks combining the use of optical fiber and coaxial cable. The fiber optic portion of such a network extends from the headend to a hub and/or to a fiber optic node. Various services of the operator may be encoded, modulated and upconverted onto RF carriers, combined onto a single electrical signal, and inserted into an optical transmitter at the headend. The optical transmitter converts the electrical signal to a downstream optically modulated signal that is transmitted to the nodes. The node may be connected to many network elements of subscribers via the coaxial cable portion of the network. By way of example, a single node may be connected to thousands of cable modems or other network elements.
Each node includes a broadband optical receiver which converts the downstream optically modulated signal received from the headend/hub to an electrical signal provided to the subscribers' network elements via the coaxial portion of the HFC network. Each node may also contain a reverse/return path transmitter that is able to relay communications from a subscriber to the headend. Thus, the HFC network uses optical fiber for communications between the headend and nodes and coaxial cable for communications between the nodes and the end user network elements. Downstream (also referred to as forward path) optical communications over the optical fiber are typically converted at the nodes to Radio Frequency (RF) communications for transmission over the coaxial cable. Conversely, upstream (also referred to as return path) RF communications from the network elements are provided over the coaxial cables and are typically converted at the nodes to optical communications for transmission over the optical fiber to the headend.
As an alternative to the above referenced HFC system, a cable TV operator may also use Radio Frequency over Glass (RFoG) systems to deliver the same services as an RF/DOCSIS/HFC network. RFoG and HFC systems can concurrently operate out of the same headend/hub, permitting RFoG to be a solution for node splitting and capacity increases on an existing HFC network. RFoG permits the continued use of traditional HFC equipment and back-office applications with fiber-to-the-premise deployments. Thus, use of existing CMTS platforms, headend equipment, set-top boxes, and cable modems can continue while gaining benefits inherent with RFoG systems.
In an RFoG system, RFoG optical networking units (R-ONUs) terminate the fiber connection at a subscriber-side interface and convert traffic for delivery over the in-home network at the customer premises. For example, the R-ONU may connect to set-top boxes, cable modems, or like network elements via coaxial cable, and one or more of the cable modems may connect to the subscriber's internal telephone wiring and/or to personal computers or like devices via Ethernet or Wi-Fi connections. The return path for voice, data, video, and like upstream traffic from a cable modem or like network device is through the R-ONU which converts the upstream signal to an optical upstream signal and which transmits the optical upstream signal to the return path RFoG optical receiver at the headend or hub. The RFoG optical receiver converts the upstream optical signal at the headend to an RF electrical signal for the CMTS.
Accordingly, R-ONUs convert optical signals from the headend into electrical signals at the customer premises and thereby terminate the RFoG system at the subscriber-side interface. This is accomplished in place of the same function traditionally performed back at the higher-level serving area optical nodes in the HFC network. The RF infrastructure remains in place; the difference is that the optic fiber termination is moved from an optical fiber node of the HFC network to the R-ONU at the customer premises. By way of example, the R-ONU can be located at a single home, a business, a multi-tenant dwelling (MTU/MDU) or an individual living unit within an MTU.
Although RFoG systems provide a possible capacity increase relative to traditional HFC systems, an undesired effect of an RFoG system is the potential for upstream interference that may occur when more than one R-ONU has the optical return path activated at any given time. For example, when amplitude modulation (AM) is used in the upstream path and optical upstream signals are received by the RFoG optical receiver at the headend corresponding to overlapping transmissions or bursts from multiple R-ONUs of about the same wavelength or of close wavelengths, an optical collision or optical beating may occur and cause optical beat interference (OBI). OBI is a signal degradation mechanism in systems using amplitude modulation that occurs when two or more lasers with closely-spaced optical frequencies or wavelength transmit into optical fiber and mix together in the RFoG optical receiver causing splatter in the RF spectrum. The impact of OBI is packet loss, i.e. the transmissions received via the R-ONUs cannot be properly demodulated at the headend. This may particularly be a problem if the bursts or transmissions contain voice packets. In systems that use frequency modulation (FM) in the upstream path, any overlap of transmissions from different R-ONUs may cause upstream interference, even if the transmit wavelengths are far apart.
In Data Over Cable Service Interface Specification (DOCSIS) 2.0 deployments, cable modems are capable of transmitting in only a single channel at a time (i.e., no channel bonding capability). Thus, for multiple cable modems in an RFoG system to transmit simultaneously requires the cable modems to be tuned to different RF channels. This is a common practice for purposes of increasing throughput and thus OBI and upstream interference are a potential problem. In DOCSIS 3.0 deployments, cable modems have upstream channel bonding capability and are therefore able to simultaneously transmit in different RF channels. The resulting simultaneous use of multiple channels by different cable modems in an RFoG system and increased usage of networks further enhance a likelihood of OBI, upstream interference, and modulation errors.